In a show of solidarity with Ukraine’s embattled interim government, Vice President Biden issued a warning to Russia to follow the agreement struck in Geneva to diffuse tensions. Meanwhile, pro-Russian separatists in Eastern Ukraine continued to defy the accord, and Ukraine’s acting president announced the resumption of “anti-terrorist” operations against the separatists. Gwen Ifill reports.

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GWEN IFILL:

Vice President Joe Biden met with Ukraine's leaders today and issued another warning to Russia.

VICE PRESIDENT JOSEPH BIDEN:

We have heard a lot from Russian officials in the past few days, but now it's time for Russia to stop talking and start acting.

GWEN IFILL:

Today, it was the vice president talking, on the ground in Kiev, leading a U.S. congressional delegation in a show of solidarity with Ukraine's embattled interim government. He said it's past time for Russia to live up to the agreement struck in Geneva last week to lower tensions.

VICE PRESIDENT JOSEPH BIDEN:

These are commitments made. They should be fulfilled. We need to see these kinds of concrete steps. We need to see them without delay. We will not allow this to become an open-ended process. Time is short in which to make progress.

We demand only one thing. Fulfill your international obligations. They shouldn't behave as gangsters in the modern century.

GWEN IFILL:

Pro-Russian separatists in Eastern Ukrainian cities continued to defy the Geneva accord calling for an end to their occupation of government buildings.

An official of the self-proclaimed Donetsk republic said negotiations are under way with international mediators.

OLEKSANDR KHRIAKOV, "Donetsk Republic" (through interpreter): At the moment, we have come to an agreement to vacate the session hall and two floors of the regional administrative building. I don't know the details, but we're now negotiating with the officials in order to provide conditions for them to renew their work.

GWEN IFILL:

But Ukraine's acting president announced the resumption of anti-terrorist operations against the separatists, as a top security official warned the government may have no choice but to use force.

Such actions and crimes in Eastern Ukraine by armed saboteurs, in our opinion, cynically undermines the quadrilateral agreements signed in Geneva. We retain our right to react adequately to such crimes.

GWEN IFILL:

Tensions were especially high in Slavyansk, where three men were shot Sunday, under disputed circumstances, at a pro-Russian checkpoint. They were laid to rest today.

A short time later came reports that a Ukrainian military aircraft was fired on while overflying the city.

Back in Kiev, Vice President Biden addressed legislators in Ukraine's parliament, the Rada, and said next month's presidential vote is perhaps the most important election in Ukrainian history. He also announced an additional $50 million in U.S. aid, part of which goes to reduce reliance on Russian energy sources.

VICE PRESIDENT JOSEPH BIDEN:

We can help in stabilizing and strengthening Ukraine's economy by helping you withstand the unfair economic pressure being thrust upon you.

GWEN IFILL:

The West has ratcheted up the economic pressure on Russia after its annexation of Crimea.

We will show our teeth if it is necessary and within the law. We will appeal to courts and other institutions, including the World Trade Organization dispute settlement body. The economy is global, but companies have to operate by conditions of competition, and not by the politically-motivated economic war.

GWEN IFILL:

The prime minister added that his country could counter sanctions by shifting its exports to Asian markets.

Meanwhile, in Washington, Pentagon spokesman John Kirby announced 600 U.S. troops are being deployed for exercises in four Eastern European and Baltic NATO states.

REAR ADM. JOHN KIRBY, Pentagon Press Secretary:

Since Russia's aggression in Ukraine, we have been constantly looking at ways to reassure our allies and partners.

GWEN IFILL:

And this morning, the guided missile frigate USS Taylor made its way through the Bosporus Straits on its way into the Black Sea, which touches the coastlines of Ukraine, Russia and Crimea.